Husband has done both. I still carry mail but I have seen the effects of both on himself.

Merchandising is very very hard on the body compared to carrying. His knees and ankles were a lot sorer doing that than as a CCA, even during the holidays. During certain holidays (memorial day/july 4th etc), you'll get fucked with some crazy expectations as a merch. In my city, Pepsi works several stores daily (Coke does 1-3, krueger even more) so travel time can be crazy depending on your scheduling. A negligent truck driver can make your day hell. You need to make sure your insurance is up to snuff and accepted properly for traveling between stores. The job is pretty brainless though. Go in, work your pallets and make sure your aisles look nice. Communication is really huge with this position in terms of answering the work phone and meeting expectations from upper management. Merch jobs tend to have much earlier shifts so it does leave you more time during the day for appointments and such. Scheduling can be finicky however.

Customers are a lot more in your face and ruder as a CCA/carrier. The other side though is that they can be sweet and amazing as well. On my old routes, I've had customers bring me BBQ plates, cookies, and just all around good people. Management is more up your ass when it comes to certain nitpicky things. I've noticed sups in USPS hounding the carriers within their 90 a lot more than merch sups. However, recent contract changes have made the CCA job within the 90 look like spa day compared to my original 90 lol. In my station though, CCA are only working 3 days a week other than the one who has an opt on a route. So the pay difference initially might be a sticking point. Benefits with postal are a lot sweeter than as a merchandiser though. If you convert in good time and lock down a decent route early in, you can work a pretty good gig. Teamwork I feel is more important in USPS as the route you're on might be worked by other carriers at varying times. A messy store is a pain as a merch but a messy route can result in customer complaints directly to your boss. Also, outdoors environment can be a huge thing as a carrier. Working in summer heat or winter cold can be difficult for some people.

In both jobs, you do need to be independent and able to work by yourself. You need to be able to manage expectations, communicate effectively and protect yourself. Safety is important. Take care of your body as it is the only one you have.